'baddish' is formed in English by adding the suffix '-ish' to the adjective 'bad'; 'baddishly' is then formed by adding the adverbial suffix '-ly' to 'baddish'.

'bad' appears in Middle English (forms like 'badde'/'bad') and developed into colloquial/adjectival forms such as 'baddish' (XIX–early XX century usages in informal speech), which in turn produced the adverb 'baddishly' by regular suffixation.

Originally 'bad' denoted something morally or qualitatively poor; 'baddish' came to mean 'somewhat bad' or 'having bad-like qualities', and 'baddishly' now means 'in a bad or wicked manner' (often with a slightly colloquial or ironic tone).